1. Field of Use
This invention relates generally to hydrostatic transmissions for the swing mechanisms of machinery, such as hoes, cranes, shovels or the like, wherein the transmission comprises a main hydraulic pump coupled in a closed loop circuit to a hydraulic motor. More specifically, it relates to control systems for such transmissions wherein lever operated control valves selectively direct control fluid from a charge pump to the main pump stroking pistons to control motor speed and direction; wherein torque feedback signals from the circuit modulate the control valves to thereby control motor torque; and wherein balancing signals from the charge pump balance the torque feedback signals to render the control valves insensitive to variations in charge pump pressure.
2. Description of the Prior Art
Some hydraulic power transmission systems for accelerating, driving and braking high inertia loads, such as the swing mechanism of a power shovel or crane, employ a hydrostatic transmission comprising a hydraulic pump and a hydraulic motor. In such systems the load is best controlled by regulating motor torque rather than motor speed. U.S. Pat. No. 3,747,350 discloses an open loop power transmission system which uses a directional valve disposed between a fixed displacement pump and a unidirectional motor and a pilot operated relief valve for controlling torque, as well as other motor functions. The present invention is an improvement over a system disclosed in Bulletin 9565 Rev. D dated June, 1972 entitled "Heavy Duty Transmissions" of Sundstrand Hydro-Transmission, Ames, Iowa 50010.
It is an object of the present invention to provide a closed loop torque control system which avoids the use of directional control valves between the pump and the motor, which avoids dependence on energy-wasting relief or dump valves for system control, which is insensitive to variations in charge pump pressure and offers other advantages.